feat: update terser to version 4

[alan-agius4]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

Breaking Changes

Additional Info

Full terser changelog https://github.com/terser-js/terser/blob/master/CHANGELOG.md#v400

[codecov]

Codecov Report

Merging #94 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #94   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files           5        5           
  Lines         283      283           
  Branches      113      113           
=======================================
  Hits          278      278           
  Misses          5        5

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[alexander-akait]

Ci fail

[alan-agius4]

The failure doesn’t seem related https://dev.azure.com/webpack-contrib/terser-webpack-plugin/_build/results?buildId=153

[alexander-akait]

No https://dev.azure.com/webpack-contrib/terser-webpack-plugin/_build/results?buildId=153

[alexander-akait]

/cc @fabiosantoscode it is safe to release as minor version, not major (we require node@6)?

[fabiosantoscode]

@evilebottnawi The internal AST has changed. If you allow your users to access the internal AST like this then it's a major change.

[alexander-akait]

Thanks, we don't allow developers use ast so it is minor release, thanks